Conventional fishing trawl can be compared to a big funnel made from net-meshes of different sizes. It has a big opening at one end and narrows down into the trawl bag at the other end where the fish is collected. The trawl is towed after the fishing vessel. Big trawl doors connected by warps to the vessel, floats and bobbins fastened to the fishing line and the foot rope keeps the trawl open. Marine species in the track of the trawl opening that can not swim faster than the trawl is towed by the vessel are caught in the trawl and finally land in the trawl bag.
One of the pre-requisition for sustainable fishing is that governments grant individual quotas, most often to fishing vessels, based on total allowable catches of individual species in their territorial waters. The granted quota, specifies marine species and quantities allocated to the vessel. A vessel is not permitted to land species not included in its quota, unless it is a non quota species. The same is valid for fishes below certain size or weight. The regulations for allocations of quoata can vary from one country to another between fishing grounds and seasons.
Conventional fishing trawl is not designed to differentiate between fish species and it can be difficult to guaranty that fish below certain size does not land in the trawl bag. Mesh sizes in the aft-part of the trawl can deform considerably when fish is collected in the bag and it can then be difficult for small and juvenile fishes to escape through the net meshes. All this results in that small and non quota species are thrown back into the sea. Fish that lands in trawl bag is most often damaged in their scale beyond healing.
In recent years increased interest has been on developing fishing gear that could sort out marine- species that are not to be caught. Such fishing gear would support sustainable development of the marine resources of the sea.
Different equipment's are known for sorting out fish sizes and species not to be caught when trawl fishing. As an example one can mention U.S. Pat. No. 4,739,574, from 1988 that concerns a turtle exclusion device inserted in the trawl body of a shrimp trawl in front of the trawl bag.
Danish Patent No. 170.355, relates to method and equipment to hold fully open meshes in the aft-body of a trawl.
Icelandic Patent No. 1605, laid forward Jun. 20, 1996, describes a device inserted in a trawl in front of the trawl bag. The device consists of two outslip unit units and one stop unit. The grid frames each being with pre-determined number of rods. The outslip unit units are inserted in such a way that they form part of the outer net surface of the trawl and the first grid forms ca. 23.degree. inclination in the direction to the trawl bag where as the other grid is parallel or almost parallel to the trawl axis. The stop unit extend from the end of the second grid to the upper part of the trawl. Fish caught in the trawl moves aft into the trawl and lands on the first grid unit and fish that has smaller diameter than the distance between the rods can escape. It is however a disadvantage of this device that only part of the fish lands on the grid units due to the relativity low water velocity induced by the placement of the grid units.